The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A network device generally comprises a physical layer (PHY) module and a medium access controller (MAC). In wired networks, the PHY connects the network device to a cable. The MAC provides an interface between the PHY and a host. Referring now to FIG. 1, a first network device 20 is connected to a second network device 22 using a cable 14. In a typical network, the first network device 20 may be called a local device and the network device 22 may be called a link partner of the local device, or vice versa.
The first network device 20 comprises a PHY 24 and a MAC 26. The PHY 24 is coupled to the cable 14 via a connector 12. The second network device 22 comprises a PHY 28 and a MAC 30. The PHY 28 is coupled to the cable 14 via a connector 15. In an Ethernet-based network, the cable 14 is generally a CAT 5 or CAT 6 twisted-pair cable. The connectors 12 and 15 are generally RJ45 connectors.
A PHY of a network device may communicate at different speeds and use different modes of communication depending on the topology of the network. For example, the PHY of the network device that operates in a Fast Ethernet network may communicate at 100 Mbps (Mega bits per second) while the PHY of the network device that operates in a Gigabit Ethernet network may communicate at 1000 Mbps (i.e., 1 Giga bits per second). Additionally, the PHY of the network device that communicates using a full duplex mode can transmit and receive data simultaneously while the PHY of the network device that communicates using a half duplex mode can only transmit or receive data at a given time. Consequently, data transfer rates are usually higher in full duplex mode than in half duplex mode.
PHY's of some network devices use a mechanism called auto-negotiation to exchange communication mode information. For example, PHY 24 of the local device 20 may advertise the communication mode of PHY 24 to PHY 28 of the link partner 22. Additionally, PHY 24 may detect the communication mode of PHY 28. Thereafter, the communication modes of PHY 24 and PHY 28 may be synchronized.
Specifically, auto-negotiation is a mechanism that takes control of the cable 14 when a connection is established between the PHY 24 of the first network device 20 and the PHY 28 of the second network device 22. Using auto-negotiation, the PHY 24 advertises the communication modes in which PHY 24 can communicate and detects the communication modes in which PHY 28 can communicate. Thereafter, auto-negotiation automatically synchronizes the communication modes of PHY 24 and PHY 28 so that the first network device 20 and the second network device 22 may communicate at the highest possible data transfer rate.
PHY's of some network devices, however, may be incapable of auto-negotiation. For example, PHY 24 may be capable of auto-negotiation, but PHY 28 may be incapable of auto-negotiation. In that case, PHY 24 uses a mechanism called parallel detection and communicates with PHY 28 in half duplex mode according to the IEEE 802.3u standard, which is hereby incorporated by reference in its entirety.